Correction for spherical and chromatic aberrations in electron lenses



June 4, 1946. E G, RAMBERG 2,401,315

CORRECTION FOR SPHERICAL AND CHROMAT-IC ABERRATIONS IN ELECTRON LENsEs Filed Sept. 26, 1942 (Ittorneg Patented June 4, 1946 MATIC ABERRATION S LENSES IN ELECTRON Edward G. Bamberg, Feasterville, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application September 26, 1942, Serial N0. 459,814

(Cl. Z50- 161) 9 Claims.

systemsv and particularly to the correction of spherical and chromatic aberrations of electron lenses.

Heretofore various methods have been devised for correcting only for chromatic aberration in electron lenses. Such systems have included. no means for correcting simultaneously for inherent spherical aberration. In an article by Otto Scherzer in Zeitschrift fr Physik, vol. 114, November 18, 1939, pages 427 to 434, entitled The theoretically attainable resolving power of the electron microscope, a system is described for correcting only for spherical aberration, in which the aperture defect (spherical aberration) is supposedly compensated with electron mirrors or screen lenses by changing the sign of the aperture defect of `the projection lens of the microscope focusing system. This method disclosed by Scherzer has notbeen found very practical.

The instant invention contemplates the simultaneous correction for both chromatic and sphericalV aberrations of an electron lens by subjecting an electron beam to a decelerating electron field within the focus of the lens to be corrected.

The simultaneous correction for bothtypesv of aberration has been found to be dependent upon a predetermined relationbetween the depth of the electron decelerating field and the focal length of the-lens to be corrected. While simultaneously complete correction of both spherical and chromatic aberrations is diicult in practice, complete correction of spherical, and partial correction of chromatic aberration is feasible by the method and means herein disclosed for an electron objective lens. Likewise, the method. and means disclosedmay be utilized for correcting only for spherical, or only for chromatic aberration.

Among the objects of the invention are to pro.-

vide an improved means for and method oi correcting simultaneously for spherical and chromatic aberration in an electron lens system. Another 'object is to provide an improved means for and method of correcting simultaneously for vided by thedecelerating eld. Another object is toprovifde an improved means for and method of 'Correcting spherical andchromatic aberrations of an electron lens wherein electrons irradiating an Vobject aresubjected to the action of a decelerating eld which reflects electrons received from the object and applies them to an electron lens. A further object Vof the invention is to provide an improved means for and method of correcting for 'sphericai and chromatic aberrations of an electron lens'wherein a source of electrons is focused upon an object, electrons'transmitted by the' object `are`subjected to a decelerating field, and said fiel'd reflects said transmitted electrons back throughthe electron lens, whereby the inherent aberrations of the lens are substantially neutralized' by the aberrations of the decelerating field. lAnother object is to provide a curved electrodefor forming one boundary of the above mentioned decelerating field. "The invention will be described by reference to the accompanying drawing, of which Fig. 1 is a schematic diagram for the purpose of explaining theoretically the operation of the system; Fig. Zi's aVV schematic diagram of 'one embodiment of the invention;l and' Fig. 3 is a schematic diagram of a second' embodiment of the invention. Similar reference'numerals are applied to similar elements throughout' thedrawing.

Referring to Fig. I; an electron beam from a source I is focused by an electron objective lens 2 or an object 3 disposed in the eld thereof. The positive vterminal of a source of potential V is`connected to the object 3. The negative terminal ofthe potential source V is connected to a plane electrode 4 disposed in the path of the electron beam and in a plane BB substantially normal thereto. The negative terminal of the potential source Vis also connected to the electron'"source lj. The distance d between the elec- ,trdef`Y A :and the object 3' is somewhat less than iof the focal length of the objective 'lens 2. The electric field 'formed between the object 3 and the decelerating electrode 4 provides an electron mirror. A virtual image. of the object will be formed. in the plane FF' at a distance 3d from the plane BB of the decelerating electrode 4.

The spherical aberration (the radius of the circleof confusion about the image point P at the intersection of the plane FF with the lens axis PP) is Y (1) Ars=4d03 where 0 is the angle of inclination of the incident electrons to the axis at the object and image.

Similarly, the chromatic aberrations of the decelerating field (the circle of confusion for electrons of kinetic energy V-i-Av at the plane AA of the object 3) is If we suppose that the focal point of the lens 2 falls in the plane FF (that is, the focal length is somewhat larger than 4d) the spherical aberration of the lens will be (3) Arm-cia3" where C may be any value larger than one-quarter. (See R. Rebsch, Ann. d. Physik 31, 551, 193.8.)

Likewise, the chromatic abberation of the electron lens will be Where K is less than one' but Very close to unity. Since f is slightly larger than 4d, it is apparent that the aberrations may be made to substantially neutralize each other.

Referring to Fig. 2, a source of electrons, comprising a cathode I l,irradiatcs an object 3,'comprising a substantially'opaque material supported by a conducting film I3. The positive terminal of the source of potential V is connected' to the object support I3. The negative terminalV of the potential source V is connected to a reflecting electrode 4 disposed substantially normal to the beam of the emitted electrons and in the same plane as the electron emitting cathode II. The distance d is derived in the same manner as described for the theoretical system of Figi. Electrons striking the object 3 are reflected therefrom in the decelerating field between the object support I3 and the reflecting electrode Ilv and are again reflected by the electron mirror, formed by the decelerating field, away from the cathode II and reflecting electrode 4. The last mentioned reflected beam passes through the object support I3 and is focused by the electron lens 2 to form a desired image. It is to be understood that the electron lens 2 may be of either the electromagnetic or electrostatic types well known n the art. When the distance d, between the object and the reflecting electrode 4, Vis suitably related to the focal length of the electron lens 2, the resultant spherical and chromatic aberrations of the decelerating field will substantially neutralize the inherent spherical and chromatic aberrations of the electron lens 2. The system described has particular utility in 'the observation of substantially electron-opaque surfaces. In a system of this type, correction for chromatic aberration is particularly desirable.

Fig. 3 comprises a system, similar to the theoretical system described in Fig. l, wherein an electron source comprising a cathode IIv is for cused by an electron lens 2 upon an electron permeable object 3 in the field of the lens. The positive terminal of a source of potential V is connected to the object 3 through the object support I3. The negative terminal of the potential source V is connected to a reflecting electrode 4 disposed at a distance d from the object and substantially normal to the axis of the electron lens. 'Ihe negative terminal of the potential source V is likewise connected to the cathode Il. Electrons transmitted by the object 3 are reflected by the reflecting electrode 4 by means of the decelerating field between the object 3 and the reflecting electrode 4. 'Ihe reflected electrons are again transmitted either by the object 3, or its support -flected electrons.

I3, and again focused by the electron lens 2. In a system of the type of Fig. 3, correction for spherical aberration is primarily important, While correction for chromatic aberration is of secondary importance.

In-each of the systems;describedfheretofore, the correction forr aberrations Vmay be 'somewhat improved by curving the reflecting electrode 4, as shown in Fig. 2, to provide additional cornpensation for electrons at higher angles of incidence, and to adapt the aberrations of the reflector to those of a given lens. Furthermore, the distance d may be increased, if desired, by increasing its negative potential with respect; to the potential at the plane AA of the object 3. With this arrangement, the distance between the object plane and the plane of reversal of the electrons becomes somewhat less than 1/4 of the focal length of the lens, While the distance d is merely dependent upon the value of the poten tial V. i 1

Thus the invention described comprises a method of and means for correcting Vsimultaneously for spherical and chromatic aberrations of an electron lens, by subjecting electrons focused by the lens to the compensating effects of electronic arrays of the required strength and phase derived from an electron decelerating field. Substantially perfect correction of the chromatic and spherical aberrations of the lens may. be obtained by a predeterminedrelation between the depthrof the decelerating fieldand the focal length .and aberration constantsof the electron lens. i

I claim as my invention:

l. In an electron beam focusing system including an electron lens and a decelerating electric field, the methodof correcting simultaneously for spherical and chromatic aberrations comprising adjusting the depth of said field to have a predetermined relation to the focal length of said lens, directing said beam through saidilens to focus saidv beam, applying said focused .beam to said field to reflect said beam, and directing said reflected Vbeam again through said lens to form electronic arrays to compensate for inherent aberrations in said lens.

2. In an electron beam focusing system include ing an electron lens and a decelerating electric field, the method of correcting'simultaneously for spherical and chromatic aberrations comprising adjusting the depth of said field to have `a predetermined relation vto the focal length of said lens, directing said beam through said lens to focus said beam, applying said focused beam to said field to reect said beam, and directing said reected beam again through said lens to 'form electronic arrays of the density and phase required to compensate for inherent aberrations in said lens.

- 3. In an electron beam focusing system for irradiatingan object including anv electron lens and a decelerating electric eld, the -method of correcting simultaneously for spherical and chromatic aberrations comprising adjusting the depth of 4said field to have a predetermined relation to the focal length of said lens, focusing saidv beam to penetrate said object, subjectingv electrons transmitted by said object to said decelerating field to reflect'said electrons and applying said reflected electrons to said lens to focussaid refi. In an electron beam focusing system for fr.- radiating an object includlng'an electron lens and a decelerating electric field, the method-of correcting simultaneously for spherical and chromatic aberrations comprising adjusting the depth of said field to have a predetermined relation to the focal length of said lens, focusing said beam to penetrate said object, subjecting electrons transmitted by said object to said decelerating eld to reflect said electrons and to form an electronic image having aberrations of equal value and opposite sign to that formed by inherent aberrations in said lens.

5. Apparatus for correcting simultaneously for spherical and chromatic aberrations of an electron beam by an electron lens including means for focusing said beamy means for forming a decelerating electric field for reecting said beam, means for adjusting the depth of said field to have a predetermined relation to the focal length of said focusing means, means for directing said beam through said focusing means to said decelerating field to form a reflected electronic image said reflected image being again directed through said focusing means.

6. Apparatus for correcting simultaneously for spherical and chromatic aberrations of an electron beam focused upon an object including means for supporting an object, means for focusing said' beam upon said object to penetrate said object, means for forming a decelerating field for reflecting said beam, and means for subjecting electronstransmitted by said object to said field to derive electronic arrays having aberrations of equal value and opposite sign to the electronic array formed by the aberrations in said focusing means.

7. Apparatus of the type described in claim 6 including means for adjusting the depth of said field to have a predetermined relation to the focal length of said focusing means.

8. Apparatus for correcting simultaneously for spherical and chromatic aberrations of an electron beam focused upon an object by electron focusing means including means for supporting an object, electromagnetic means for focusing said beam upon said object to penetrate said object, means for forming a decelerating field for reflecting said beam, and means for subjecting electrons transmitted by said object to said field to derive anelectronic image having aberrations of equal value and opposite sign to aberrations in said focusing means.

9. Apparatus for correcting simultaneously for spherical and chromatic aberration of an electron beam including means for supporting an object, means forfocusing said beam upon said object to penetrate said object, means including a curved reflector and a source of potential connected between said reflector and the plane of said object for forming a decelerating field for reflecting said beam, and means for subjecting electrons transmitted by said object to said field to derive an electronic image having aberrations of equal value and opposite sign to aberrations in said focusing means.

EDWARD GRAMBERG. 

